Thermal safety pilot switch



Nov. 14, 1950 s. R. HOOD 'I'l-IERMAL SAFETY PILOT SWITCH Filed Nov. 14, 1946 153 INVENTOR.

Patented Nov. 14, 1950 UNITED STATES PATENT OFFICE 1 Claim.

My invention relates to thermal safety pilots particularly adapted for use with automatic gas burning equipment.

In gas burning equipment, it is common to employ a constantly burning pilot flame for igniting a main burner operated on a full-off andfullon cycle. The gas may be entirely pre-mixed before delivery to the burner or it may have some primary air with the balance of the secondary air admitted at the burner. Whatever the burner design may be, there is considerable danger of explosion in the event gas should be delivered to the furnace at a time when the pilot flame is extinguished. Safety, therefore, requires the use of a device for preventing delivery of gas to the furnace when the pilot flame is extinguished and several forms of device have been suggested and employed for the purpose. Safety pilots employing thermostatic metal have been used in the past but due to the existence of problems thought to be inherent in thermostatic metal devices, and also because of increasingly severe safety requirements of local ordinances and testing laboratories" specifications, thermostatic metal operated controls have been replaced very'largely by other forms of controls known in the art. 7

Among the requirements of a. thermal safety pilot are the rapid closing of contacts after the pilot has been lighted, but, more importantly, quick separation of the contacts on pilot failure. Thermostatically operated controls have not heretofore fully satisfied these requirements due, in part, to conditions of use, characteristics of bimetal and limitations of design. In order to obtain quick closing of contacts they must be placed relatively close together, but when so placed problems have been encountered which have not been overcome. If the stationary contact is rigidly mounted, development of a relatively high temperature, or continued subjection of the bimetal to an intermediate temperature, would frequently introduce a permanent strain in the bimetal which would impair its functioning and cause ultimate failure. To overcome this problem, safety pilots have employed a spring supported contact against which the bimetal expands, and while the use of a spring inthis manner decreases the possibility of strain in the bimetal, it increases markedly the time required for opening of contacts on pilot flame failure. Problems have also occurred due to the tendency to shear the bond between the high and low ex pansive metals, particularly when a relatively great temperature differential is encountered.

The principal object of my invention is the provision of a thermal safety pilot ofthe thermostatic metal operated type which can be em- 2 p-loyed satisfactorily regardless of the severity of safety requirements, and will have long useful life.

Another object is to overcome the problems outlined hereinabove in the use of thermostatic metal operated controls.

In accordance with the main features of my invention, I employ two bimetal elements, each carrying a suitable electrical contact, with one bimetal element heated by the pilot flame, and both positioned to deflectifi 're's'ponse to ambient temperature changes, the bimetal elements and contacts being so positioned relatively to each other that heating of the one element from the pilot flame closes the contacts and the development of a higher ambient temperature in response to lighting of the main burner, for example, deflectsboth bimetal elements in the same direction to maintain substantialy the same contact pressure as that created merely by the operation of the pilot flame. The two elements of the bimetal preferably have a high creep strength and a relatively low difference in thermal expan sion rate, preferably being less than IO l0 per degree centigrade. The safety pilot also employs a simplified structural arrangement as will be pointed out in the detailed description which follows.

Other objects and specific features of the invention will be pointed out in connection with the'detailed description taken with the accompanying drawings in which Fig. 1 is an elevational view of the safety pilot mounted in position, with a shield shown in section to facilitate showing of the thermostatic metal elements, and with the circuit arrangement employed with the pilot shown schematically;

Fig. 2 is a sectional view aken on the line 2-2 of Fig. 1, looking in the direction of the arrows;

Fig. 3 is a plan sectional view taken on the line 33 of Fig. 1, but with the flame guard removed in order more clearly to show some of the structural parts;

Fig. 4 is a perspective view showing the flame uard removed from the pilot frame;

Fig. 5 is a perspective view of the mounting and supporting frame;

Fig. 6 is a fragmentary transverse sectional view taken on the line 65 of Fig. 3 looking in the direction of the arrows;

Fig. 7 is a fragmentary View of the pilot flame tube; and. a I

Fig. 8 isa View similar to Fig. l but showing a modification.

Looking now first to the embodiment shown in Figs. 1. -7, inclusive, the safety pilot of my invention utilizes a frame member ID formed of Monel metal or like corrosion resistant metal which comprises the entire support for the parts of the device and also affords the means for mounting the safety pilot in proper position in a. furnace. Mounting of the device is facilitated by the provision of a pair of ears II of the type shown. The device comprises essentially a pilot light tube I2 and a pair of bimetal elements I3 and I4 carrying electrical contacts I6 and II. To support and insulate the bimetallic elements I3 and I4, I provide three porcelain blocks I8, I9 and H, each of which is provided with a vertically disposed channel in which the bimetallic elements are supported. The porcelain blocks may be identical in shape, and by facing v the channels of the blocks I8 and I9 toward each other they provide sufficient channel space to receive the relatively thicker strip I3; while the relatively smaller channel provided by the single block 2| is suflicient to receive the somewhat narrower strip I4. As indicated in Fig. 6, means is provided for preventing longitudinal movement of the strips with respect to the porcelain blocks, and I have found a simple means to comprise an integral projection 22 on the strip, which projection is adapted to extend into a slight indentation (not shown) formed in the porcelain. This locks the strip in position, but those skilled in the art will understand that any suitable means for locking the strips against longitudinal movement may be employed.

As may be seen, particularly by reference to Figs. and 6, the legs of the U-shaped frame are channeled in order more positively to enclose the porcelain blocks and strengthen the frame, and, also, are provided with top and bottom generally triangular edge extensions 23. The pilot tube has a mounting bracket 24 intermediate its ends, between the top and bottom extensions of which the projections 23 engage as shown particularly in Fig. 3. A single assembly bolt 2'5 is employed between the legs of the U frame, with the result that when the parts are mounted in the position shown particularly in Fig. 3 and the nut tightened on bolt 25, the slanting sides of the triangular projections 23 will force the bracket 24 toward the assembled porcelain blocks and firmly engage the bracket 24 and porcelain blocks I8, I 9 and 2| between the legs of the U and in firm engagement with respect to each other. The assembly is, therefore, firmly supported by means of the single nut and bolt fastening means and all of the parts are effectively protected against dislodgment regardless of which direction force may be applied.

. The tube I2 is provided with suitable means for attachment to a source of vaporous fuel and suitable means is provided for the introduction of primary and/or secondary air, depending upon the particular type of installation employed. The tube is suitably provided with a combustion orifice to deliver a flame 21 toward a main burner (not shown) to light the same when vaporous fuel is delivered thereto. The pilot tube is also provided with a small orifice 28 which causes a flame to be projected in a direction to apply heat to the bimetal element I3. Preferably, a narrow slot is provided along one side of the tube to form a connection between the main gas discharge orifice and auxiliary orifice Z8 utilized to impart heat to the bimetal element. Those skilled in the art will understand that modified arrangements may be made whereby to cause heat from the pilot flame to be imparted to the bimetal element for the intended purpose. In the arrangement shown, I preferably utilize a guard 3i which may be formed of two identical parts, as shown in Fig. 4, riveted together along their edges, the entire guard being so proportioned that it may be clipped in removable fashion around the outside of the frame I0.

Suitable means is provided for connecting the bimetal elements I3 and I4 with their contacts I6 and II, respectively, into a suitable control circuit. In the drawing, I show screw terminals 32 and 33 on the bottom projecting ends of the bimetal elements, it being noted that one of the projecting ends is bent at an angle to make the screw 32 accessible while still employing a minimum length of bimetallic strip material below the porcelain blocks at a point where it does not perform a function. The control circuit may vary but in the drawing I indicate a simple conventional type of arrangement employing a room thermostat 34, a furnace limit switch 36, a solenoid valve 3? and a secondary 38 of a low voltage transformer. In this form of circuit, when the room thermostat calls for heat, the solenoid valveopens to supply gas to the main burner. If, however, the safety pilot has been accidently extinguished since the initiation of a preceding heating cycle, the contacts I6 and I! will be separated, the solenoid valve will not be energized and dangerous unburned gases will not be introduced into the furnace through the main burner. The limit switch contacts are not of significance so far as the present invention is concerned, the limit switch being shown only for the purpose of illustrating a typical conventional type of instal ation. It may. be noted, however, that under the circumstances postulated, the furnace limit switch contacts would be closed since they open only when an excessive furnace bonnet temperature is reached, and before the demand created by the room thermostat is satisfied. It is, therefore, obvious that whenever the room thermostat initially responds to lowered room temperature, fuel will be delivered to the burner except for the provision of a safety feature such as that provided by a suitable thermal safety pilot.

My invention accomplishes both a more rapid closing and opening of the contacts due to the fact that only a small contact gap is required and an ambient compensating bimetal element I4 is pro. vided operating in parallel with the control bimetal element I3. Since the two bimetal elements are at the same temperature when the pilot light is lighted and there is only a small gap between the contacts I5 and I1, bimetal element I3 responds to quickly close the gap between the contacts when the pilot flame is ignited. At the instant the room thermostat calls for heat, the main gas valve opens and the main burner is ignited. This raises the temperature surrounding the pilot and a further deflection of the control element I 3 takes place. Normally, this additional deflection would build up a stress in this element through restraint of deflection, but in the case of my invention the compensating element I 4 is also deflected in the same direction and at the same rate, thereby maintaining the original contact pressure. If, now, the pilot flame should be extinguished for any reason, the two bimetal elements would be at the same ambient tempera ture, and the control element, no longer being heated by the pilot flame, would quickly respond in a direction to separate the contacts I6 and I1. Any change in ambient temperature, such as will occur if the main burner is extinguished when the demand for heat no longer exists, will result in a parallel deflection of the two elements, thereby maintaining the separation of the contacts as the ambient temperature is decreased. The result is that regardless of the ambient temperature a very rapid breaking of the circuit by the separation of the contacts and H can be made to take place in event of pilot flame failure.

In my study of the thermostatic element safety pilot, I have determined that malfunction results from still another reason due to mechanical overstressing as a result of excessive temperatures or heating at moderate temperatures for extended periods of time. This has been due to a shear or partial shear of the bond between the high and low expansive metals of the bimetallic strip and has been due, in part at least, to improper selection of the metals employed. Such selection has been dictated, to be sure, by attempts to overcome other problems associated with the use of bimetallic strip, which problems I have overcome in another manner not heretofore employed so far as I am aware. From the standpoint of materials, it has been common to employ for the low expansion side, a low expansive nickel steel comprising, for example, 35% to 50% nickel, balance iron. This type of alloy has low strength at high temperature and when combined with a high expansive metal of the general character of 18% chromium and 8% nickel stainless steel to form a laminated metal. The relatively large difference in thermal expansion rate and resulting relatively high expansion differential and warping rate, while advantageous for many purposes has the disadvantages in that there is a tendency to shear the bond between the high and low expansive metals. The shear stress is directly proportional to the difference in the expansion rate of the two metals. I have found that the device of my present invention, since it may utilize a very small gap, requires only a very small amount of warpage and, therefore, instead of using two metals having great difference in expansion rates, I have used two alloys having expansion rates which are only slightly different from each other and which also have a very high creep strength. By this combination shear stress is reduced to a minimum and the ability to withstand such stress is greatly increased. The alloys which may be employed for my purpose are, for example, Hastealloys or Westinghouse Electric and Manufacturing Companys K-42-B, or similar alloy, having high strength properties at elevated temperatures. I prefer to employ a bimetallic strip the two laminations of which have a difference in thermal expansion not greater than 10 x 10 per degree centigrade.

In addition to the advantageous structural features which I employ, it will be clear from the above that I provide in the thermal safety pilot of my invention two bimetal elements which operate in the same direction and at the same rate of deflection in response to force at ambient temperature, but one of which, in addition, closes a pair of electrical contacts when heated by the pilot flame; that the gap between the contacts can, as a result, be made very small and the contacts are quickly closed when the pilot flame is ignited; that since the contact pressure remains the same at all ambient temperatures, extinguishment of the pilot flame at any time will cause a quick breaking of the contacts and operation of a resulting control circuit to prevent delivery or gas to a main burner; and that,

finally, a relatively less active bimetallic strip may be employed in which the metals utilized have a high creep strength to withstand shear stresses and relatively small expansion differential whereby to reduce to a minimum the shear stresses actually applied, all without reducing the effectiveness of the operation of the device in any way.

In Fig. 8, I show a modification in which instead of employing a single bimetallic element 13, I employ a mono-metallic element H3 to which is secured a bimetallic element H3. The remaining portions of the device are in principle unchanged, and, for convenience, the same numerals are applied as in the main embodiment with, however, the prefix l to denote modification. In this form, the two bimetallic elements are affected by ambient temperature changes in exactly the same manner described hereinabove, but instead of projecting the pilot flame directly against the control element, it is projected against the mono-metallic element H3 and thence by direct conduction to the bimetallic element H3. The mono-metallic element H3 may take various forms, preferably, however, being offset in the manner shown to permit free movement of the bimetallic element as well as a substantial area of contact for the direct propagation of heat from the pilot flame through the mono-metallic element to the bimetallic control element. The element H3 is preferably formed of any suitable corrosion resistant material such as chrome steel. The operation of the device of Fig. 8 can be identical with that described in connection with Fig. 1. The top loop in the monometallic element protects the contacts, in the same manner as in the main form, and the user cannot tamper with the contacts by means of a screw driver or like tool.

The features and scope of the invention are defined by the claim.

What I claim as new and desire to protect by Letters Patent of the United States is:

A thermal safety pilot comprising a relatively long U-shaped frame, open at one end, and having a pair of mounting ears at such open end, the cross section of the U members being channel shaped, except near the mounting ears where top and bottom triangular extensions are provided, a plurality of porcelain blocks between the legs of the U, a pair of bimetallic members supported between the said blocks with lower, portions to receive conductors and upper portions shaped to extend close to each other when their temperature is uniform, contacts in mutually engageable position, a pilot tube disposed between a porcelain block and said triangular extensions and an assembly screw between legs of the U at the open end thereof to squeeze the porcelain blocks and clamp the pilot tube in position, the pilot tube having an opening to project a portion of its flame against one of said bimetal strips.

STANLEY R. HOOD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,835,993 Stark Dec. 8, 1931 1,842,336 Te Pas Jan. 19, 1932 1,880,743 Bott Oct. 4, 1932 2,098,383 Furlong Nov. 9, 1937 

